Yes, the non-baryonic dark matter component of these clusters is sufficiently large to keep the clusters somewhat cohesive. (Remember the DM component is at least 5 times larger than the total baryonic component.) I expect there would be some definite effects of getting that gas "stripped" from the clusters, but hey, the universe is a dynamic place....

Also, (I'm guessing) it is the cluster gas and dust that is colliding and getting hung up -- not so much the gas and dust within the individual galaxies.

__________________
Everyone is entitled to his own opinion, but not his own facts.

In the case of the Bullet Cluster, the universe seems to have answered your question with a resounding NO.I have no idea what this question means - as far as we can tell, the 'NBDM' is not concentrated in the galaxies in clusters, it's spread throughout the cluster, with higher DM concentration in the centre (see post #29 in this thread for more details).I see that Squashed and Cougar have already addressed most of your questions (and points), so let me add just one thing: what is 'dust'?

Answers to this - seemingly innocuous - question help constrain the nature of baryonic DM.

The dust that we can see, and which we may have already captured 'unharmed' (in Stardust), is predominently 'metals' (by mass - remember that astronomers have a very odd definition of 'metal' - any atomic species other than H or He). Yet the elemental composition of the luminous mass we have observed (and a great deal of that which we 'see' only by absorption) is overwhelmingly 'non-metal' (i.e. H and He); take your pick of the %, by mass, that metals comprise - 1%, 3%, 5% - it doesn't matter, for the purposes of this analysis.

So unless the elemental composition of the non-luminous baryonic DM is radically different from that of the luminous mass, 'dust' cannot comprise more than a tiny fraction of DM of a rich cluster (remember that the known and inferred stars, gas, and dust of galaxies comprise only a few percent of the total cluster mass - see post #36 in this thread for more details).

But can't 'dust' be grains of solid H, or drops of superfluid He? Well, the triple point of He is ~2K (below the temperature of the CMB), so it can't be He; the triple point of H is ~14K, so little lumps of solid H would be stable, right? After all, the ISO team found the dust was as cold as 10K!

(the answer to that question is left as an exercise for the reader).

So why can't ~90% of the baryonic mass of a rich cluster be in the form of 'cold, predominantly metals, dust', scattered throughout the IGM? Or, if not 'dust', then pebbles, rocks, boulders, mountains, asteroids, Lunas, Earths, ...?

And why couldn't there be lots of small balls of (cold) H+He gas, small asteroid size (in mass), up through rogue Neptune, Jupiters, to brown dwarfs, throughout the IGM?

(to be continued)

Well, to disrupt another of your excellent lectures, the differential absorption of different colors of light allows astronomers to recognize the effects of dust grains. But such significant absorption is not observed.

__________________
Everyone is entitled to his own opinion, but not his own facts.

Nereid,

If primordial blackholes (PBH) exist:

a.) would there be enough mass in PBHs to account for the dark matter?
b.) are PBHs considered as part of the dark matter?

I've seen some calculations on this, and, with the appropriate caveats and qualifications, I'd say the observational constraints on PBH being DM are very mild (that's a careful way of saying "yes").

The details on this are interesting - would anyone like to know more?No.

"DM" is just that - mass which is 'dark', and unlikely to be the sort of 'gas to stars' which we observe in our solar system neighbourhood.

'non-baryonic DM' is what folk sometimes mean when they write 'DM' - it just means that whatever the DM is, it can't be made of atoms, nuclei, electrons, etc (note that astronomers have a slightly strange shorthand here too - electrons aren't baryons, but electrons aren't 'non-baryonic DM' to astronomers!).

'cold DM', another term you will see, is DM that cannot be moving very fast wrt the regular matter in its neighbourhood, say up to 1000 km/sec. Neutrinos, of the kinds we've found so far, cannot be cold DM.

The term "DM" has its origin in observations; PBH has its origins in some theory or other - the twain have yet to meet.

I'm still trying to understand and would appreciate your thoughts...

Imagine a universe like our own. However, this universe only has two "things" in it.

• One is an earth mass planet of normal, baryonic matter.

• The other is a "clump" of non-baryonic DM that is the gravitational equivalent of the earth mass planet.

Both are at rest in relation to each other at the start of this experiment. Both are close enough to interact gravitationally in an observable timeframe.

How would these things react to each other? *

Would the two things move toward one another? Would the planet be pulled toward the clump, but the clump be unaffected by the planet? What would happen when their relative positions became superimposed with one another?

* Okay, there are three things in the universe - the third being a electromagnetically, gravitationally (and in all other respects) neutral observer that can see and measure both objects without affecting them...

The cosmic gamma ray background does not support PBH's as significant contributors to the missing mass: e.g.,

An improved gamma-ray limit on the density of PBHs
http://arxiv.org/abs/astro-ph/0304528

Dark Matter and Background Light
http://arxiv.org/abs/astro-ph/0407207

What if we can only see them as Neutrinos when they are hot and fast?

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

The planet and the CDM cloud would move toward each other, and the planet would pass through the cloud which would disrupt the cloud somewhat. The Planet would get to a point and fall back toward the disrupted cloud again. Eventually the system would reach an equilibrium in which the planet is surrounded by the cloud (and a significant amount of the cloud would be moving around inside the planet). This would result in the apparent mass (and surface gravity) of the planet being larger.

__________________
Forming opinions as we speak

Either I don't understand the question, or it's about as meaningful as "what if we can only not see them when they're invisible pink fairies?"

"What if ...?" questions are very good, and (IMHO) an essential part of doing science. However, they also need to lead somewhere (or come from somewhere) - either you can sharpen the question into something testable (in principle), or you already have some other framework or idea within which '... and that will look like a neutrino when it's travelling non-relativitically'.

Within the Standard Model, or some super-symmetric extension, I know of nothing like this.

Another angle: what is a neutrino? We know a neutrino when we see one because .... Now how could something be a neutrino only when it is moving non-relativistically?

The key is that the coalescence occurs because the system (planet plus blob of CDM) 'cools'.

In this case, it can cool only by gravitational radiation; without that, it would oscillate forever .... except, of course, if the blob of CDM were actually several (a dozen? a million? a bazillion??) separate particles, with a zero collisional cross section.And this is the key to the evolution of the universe ... baryonic matter 'cools' by emitting photons; when a blob of gas starts to 'collapse', due to its self-gravitation, it doesn't stop at being an 'isothermal sphere'*, it keeps collapsing, because it emits radiation.

*Think of globular clusters, or elliptical galaxies - the stars only very rarely collide, yet the GC is approximately spherical, and the distribution of the stars' velocities is just like that of gas molecules at a constant temperature. Of course, real GCs are not isothermal spheres, because real stars are not like gas molecules ...

And it may well be the case that that it would be about 50 quintillion separate particles with (next to) zero collisional cross section, but each perturbed in its motion in its own unique direction by the gravitational field of the planet passing near it. Eventually, the oscillating planet would heat up the blob into a large enough object that the oscillations would be very small.

__________________
Forming opinions as we speak

If interested in published sources of evidence for CDM, here a few mostly low tech selections from my virtual library:

Gravity lens reveals dark matter
http://physicsweb.org/articles/news/10/8/17/1[/I]
This one relates to the bullet cluster paper by Clowe, et. al.:
A direct empirical proof of the existence of dark matter
http://arxiv.org/abs/astro-ph/0608407

New evidence for a Dark Matter Galaxy
http://www.interactions.org/cms/?pid=1023641
". . . hydrogen gas in VIRGOHI 21 appears to be rotating, implying a dark galaxy with over ten billion times the mass of the Sun. Only one percent of this mass has been detected as neutral hydrogen - the rest appears to be dark matter. . . ."

Abell 2029: Hot News for Cold Dark Matter
http://chandra.harvard.edu/photo/2003/abell2029/
An example of using hot intracluster gas to reveal the presence of CDM

Abell 1689 Warps Space
http://apod.nasa.gov/apod/ap030109.html
An example of CDM detected using strong gravitational lensing

CFHT Gives First Glimpse of Dark Matter Distribution
http://www.cfht.hawaii.edu/News/Lensing/
A weak lensing study of ~200,000 galaxies depicting CDM mass distribution.

Dark matter comes out of the cold
http://news.bbc.co.uk/2/hi/science/nature/4679220.stm
A dynamical study of milky way satellites revealing some CDM properties"
". . . The speed is a big surprise. Current theory had predicted dark matter particles would be extremely cold, moving at a few millimetres per second; but these observations prove the particles must actually be quite warm (in cosmic terms) at 10,000 degrees. . . "

BBN, the CMB, and the Baryon Content of the Universe
http://astro.uchicago.edu/~tyler/omegab.html
A nice discussion explaining the significance of BBN and the CMB spectrum.

First Sound Waves Left Imprint on the Universe
http://www.space.com/scienceastronom...re_050111.html
Discussion of acoustical peaks in large scale structures

Dark Energy and Cosmic Sound
http://cmb.as.arizona.edu/~eisenste/...acousistic.pdf
More in depth discussion of the above.

[except, of course, if the blob of CDM were actually several (a dozen? a million? a bazillion??) separate particles, with a zero collisional cross section]

Congratulations, you can think outside the box! I am impressed!

And if this were the case, for neutrino's and CDM, and they did not interact with baryonic matter, their velocities would have nothing to do with being cold or hot, is this correct?

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

This part is very interesting! Does this mean that all the baryonic bodies in galaxies have CDM, going 'through' them?
And would this mean that a galaxy with a lot less stars would apperently need more DM, than a galxy that had Most of its gas used up in making the stars, so would have more CDM running through their masses?
And, If it is Cold and slow, how can it go through their bodies?
And, What is the only thing that we 'know' actually does 'go through' bodies?

[The planet and the CDM cloud would move toward each other]

Since the question you answered did NOT put the planet in a galaxy, following the orbital curves of spiral galaxy, why would you think that the CDM would affect the planet at all?
see here.
Big Bang Momentum

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

The original query said:
In the case of CDM in our galaxy, we presume that a uniform amount of dark matter is passing through all parts of our Solar System at the same time, and so it collectively has a neutral effect on the gravity we can measure locally by the orbits of things. It is possible that a small amount of CDM is actually bound to the Sun, but we haven't got any way to measure it, unless (this is doubtful) it is the reason for the Pioneer spacecraft to be slowing down.
Concerning the question why it would affect the planet at all, the query said there were two things in the universe, a planet, and a clump of CDM with the same mass as the planet. Both are at rest, and close enough to interact gravitationally. How could it not affect the planet? It has mass.

__________________
Forming opinions as we speak

CDM has to be able to go through reactive matter. They interact very weakly. That is why we call it weakly interacting massive particles. It is also why it is hard to detect. How can it do it? Several possible reasons, each connected to some model or another for what the WIMPs are made of. For neutralinos, they have a non-zero supersymmetric quantum number, and can only strongly interact with other matter with supersymmetric particles.
As to the only other thing we know that goes through bodies, I'm guessing you aren't talking about xrays, and assume you mean neutrinos. Neutrinos are hot dark matter.

__________________
Forming opinions as we speak

Yes, actually the 'clump' is what throws it off because it could be considered to be black hole like.

If the question would have said...we have two things in the universe, an earth sized planet and space full of DM...what would you have said?

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

[Neutrinos are hot dark matter]

Why is that the consensus? Because we know they travel at relavistic speed, right? So, there is a property being assigned to them based on this thinking...they must be 'hot', right?

Same thing with CDM... a property is being assigned to them, based on thinking...they must be cold...so they must be slow.

But if they 'go thru matter' and neutrinos 'go thru matter', then could they both be one and the same?

__________________
RussT
________________________________
Everything is, as it should be, otherwise, it wouldn't be!

He used the term clump, which sometimes has the connotation of being self-adhesive (but I assume he didn't intend that). I took it to mean a collection of dark matter particles. I took it to be in a spherical cloud within an order of magnitude as the size of the planet. I certainly don't see how it could have been thought of as a black hole (a few millimeters across).

If he had said two things in the universe, a planet, and a uniform distribution of dark matter particles spread evenly throughout the rest of the universe, I'd have said that the dark matter would not have been detectable without knowing more about the universe.

__________________
Forming opinions as we speak

Neutrinos don't interact with reactive matter much. They are weakly interacting. They are 'hot' in this case because they go fast enough that they are not bound to a galaxy cluster. Cold dark matter has that name only to say that it IS bound to galaxies and clusters of galaxies. Not becuase it is frosty or solid.
The argument against them being the same is that it would take a lot of neutrinos to account for all the mass of the dark matter, and that there are no known reactions that could create neutrinos with low enough energy to be bound to a galaxy. This doesn't rule it out, but it makes it pretty unlikely.

The nature of dark matter is still being explored. One of the options being studied by some groups are called sterile neutrinos. You can look these up if you're interested.

__________________
Forming opinions as we speak

Did this thread die on the vine, or get moved somewhere else?
TomT

It died.

__________________
Some try to tell me, thoughts they cannot defend,... - Moody Blues.

Neptune- The original Dark Matter.

The author feels that this technique of deliberately lying will actually make it easier for you to learn the ideas. - Donald Knuth